Tuning fork



Feb. 25, 1964 A. c. JONES ETAL 3,122,047

TUNING FORK Original Filed April 25. 1960 INVENTO ALBE T C. JONES RSFREDERICK J. LINGEL PETER B. TROUP W 5 n ne w THEIR ATTORNEY The presentinvention relates to tuning forks and more particularly to tuning forksof the electrically driven type.

The subject application is a division of application Ser. No. 24,534,filed April 25, 1960, now Patent No. 3,085,- 1 68.

Tuning forks of the electrically driven type are widely used asfrequency-determining elements in electrical oscil lation generators.One such application is in frequencyregulated power supplies used tosupply 400 cycle alternating current for powering instruments and otherelectrical devices used on aircraft. For such applications whereavailable space is usually at a premium, there is a great need forelectrically driven tuning forks which are small in size and yet high inefiiciency so that the entire power supply comprising the tuning forkand associated amplifier equipment can be mounted in a small space, forexample,

within an instrument case.

it is accordingly an object of this invention to provide an improvedelectrically driven tuning fork having structural features permittingsubstantial reduction in size.

Another object is to provide a tuning fork construction which is simple,rugged, reliable, and which can be built in quantity production at lowcost. I

A still further object of the invention is to provide anelectrically'driven tuning fork having improved efiiciency so as topermit reduction in size and cost of the associated amplifier equipmentwith which it is used.

Further objects and advantages of the invention will become apparent asthe following description proceeds.

Briefly, in accordance with one aspect of the invention, a tuning forkis provided which permits forming the fork by bending a flat rectangularstrip of magnetically permeable, resilient material into a U-shapedmember. 'bottom of the fork is rigidly secured as by spot welding to p abase having a dome-shaped projection which makes con- The tact with thefork over a small area. This construction is inexpensive andsubstantially reduces the transmission of vibration energy from the forkto the base, thereb preventing loss in eificiency.

For a better understanding of the invention, reference should be made tothe following detailed description taken in connection with theaccompanying drawings in which:

,FIG. 1 is an exterior perspective view of a tuning fork embodying thepresent invention;

FIG. 2 is a front elevation View of the tuning fork showing certaindetails of the magnet cores, the coils being shown schematically forclarity;

FIG. 3 is a perspective View partly in section showing structuraldetails of the coil mounting arrangement;

FIG. 4 is an exploded perspective view showing the relationship betweenthe coil parts and the tuning fork;

FIG. 5 is a top sectional view taken along the section line 55 of FIG.1; and 7 FIG. 6 is a schematic circuit diagram showing how the pickupand drive coils of the tuning fork are coupled through an electronicamplifier for regenerative oscillation.

Referring now to the drawings, there is disclosed in accordance with thepresent invention a tuning fork construction comprising a tuning forkindicated generally at 10 mounted on a rectangular base 11. The tuningfork 10 may advantageously be formed by bending a rectangu- UnitedStates Patent 0 lar flat strip of material into a U-shaped member havingtwo spaced parallel tines 12 and 13 connected by a curved bottom portion14. The base 11, which is preferably formed of a hard metal such assteel, has an upwardly extending projection or dome 15 to which thecenter of the bottom portion 14 of the tuning fork 10 is rigidly securedas by spot welding. Since the top or apex of the dome 15 has ahemispherical shape, the contact between the two parts is essentially apoint contact covering a small area. This construction minimizes thetransmission of vibration energy from the fork to the base and henceimproves the efiiciency or Q of the fork.

In order to prevent drift in the natural resonant frequency of the forkwith variations in ambient temperature due to changes in physicaldimensions and modulus of elasticity, it is preferable to construct thefork of a socalled thermally compensated alloy material havingconstituents selected to give, with appropriate heat treatment, apositive thermal coefficient of modulus of elasticity as well as apositive thermal coeificient of expansion. Such material is commerciallyavailable and sold under the name Ni-Span-C which is a nickel, iron,chromium, titanium, and carbon alloy comprising approximately 41 percentnickel, 51 percent iron, 5 percent chromium, 2 percent titanium, .06percent carbon (max) and the balance impurities. This material has notonly the required resilience for use as a tuning fork but also ismagnetical 'ly permeable, Which is a necessary requirement forelectromagnetically exciting the fork as will be apparent from thefollowing description.

secured to the base 11. The pickup coil 16 has a perma-.

nent magnet core 19 and the drive coil 17 has a similar permanent magnetcore 20, these cores extending transversely between the tines 12 and 13.The magnet cores are preferably formed of magnetirable steel having adegree of permeability to alternating magnetic fields and alsosuificient retentivity to retain a degree of permanent magnetization."The magnet cores are also dimensioned and positioned so that the outerends are spaced from the tines to form air gaps. The air gaps betweenthe ends of core 19 and tines 12 and 13 are identified by the numbers 21and 22, andthe air gaps between the core 20 and the tines 12 and 13 areidentified by the numbers 23 and 24.

In the interest of accurately maintaining the proper dimensions of theair gaps 21, 22, 23, and 24, it is important that the coils 16 and 17and their associated magnet cores 19 and 20 be accurately positioned andsupported relative to the tuning fork. In'the illustrated preferredembodiment of the invention, this is accomplished by providing in therectangular base 11 a groove 25 which is accurately machined to receiveand position the support 18. The center of the groove is tapped at 26ato receive a suitable mounting screw 26 for holding the support 18 in anupright position. As a further means of insuring the rigidity andalignment of the assembly, the coils 16 and 17 and their associatedmagnet cores 19 and 20 are imbedded in a suitable encapsulating resinsuch as an epoxy resin which surrounds the cores and coils as well asstuds 27 and 28 which are attached to and project inwardly from thesupport 18. It will be noted that the ends of the studs are providedwith grooves 29 which serve to anchor the resin casing when the resin(indicated by the numeral 30) hardens. The proper orientation of thecoils and cores is further assured by forming the magnet cores 19 and 20from bar stock having a square cross section, the cores being receivedin rectangular openings 31 in coil forms 32 which support the pickup anddrive coils 16 and 17.

The tines I2 and 13 of the tuning fork are excited by energizing thedrive coil 17 with alternating current, the leads 33 of which areconnected to the output of a semiconductor or other suitable typeamplifier 34 schematically shown in FIGURE 6. The pickup coil 16 isconnected by leads 35 to the input of the amplifier. By an appropriateselection of the gain of the amplifier and the phase relationshipsbetween the input and output voltages of the amplifier, the tuning forkis caused to vibrate continuously at its natural resonant frequency.

For sustained vibration the amplifier must, of course, have sufficientcapacity to supply the losses in the system. For that reason it isdesirable to have the efficiency or Q of the tuning fork as high aspossible so as to minimize the losses in the system and hence the sizeand cost of the associated amplifier. In accordance with another aspectof the present invention, the etficiency of the tuning fork is increasedby an arrangement of the magnetic circuit now to be described whichsubstantially improves the output of the pickup coil 16. T this end thepermanent magnet cores 19 and 2d are polarized so as to form magnetpoles adjacent the ends thereof which are indicated in FIGURE 2 of thedrawing by the symbols N and 5. Further, it will be noted that thepolarities of the magnet cores are selected such that the magnet polesof the core ends adjacent each of the tines of the tuning fork are ofopposite polarity. Thus, it will be noted that the north pole N ofmagnet core 19 and the south pole S of magnet core 2% are adjacent thetime 13. Similarly, the south pole S of magnet core 19 and the northpole N of magnet core are adjacent the tine 12. With this polarityarrangement the permanent magnetic flux between the magnet cores 19 and2t) is concentrated in the low reluctance path through the magneticallypermeable tines 12 and f3 and crosses the air gaps 21, 2.2, 23, and 24in the direction indicated by the arrows 36. By use of this reversepolarity arrangement the magnetic fields in the circuit path indicatedare in series aiding relation. This substantially increases the fluxthreading the pickup coil 16 and this, in turn, increases the output ofthe coil and eficiency of the system.

In operation it will be understood that when the polarity of alternatingcurrent supplied to the drive coil 17 by the amplifier -34 is in onedirection, the magnetomotive force supplied by this coil is in the samedirection as the magnetic field provided by the permanent magnet core 20so that these fields aid each other, and the resulting increase in thestrength of the field in the magnetic circuit through the tines causesthe tines to draw together and approach the ends of the magnet cores.This causes a decrease in the air gaps 21, 22, 23, and 24 and thereluctance of the magnetic circuit, and this causes resulting increasein the magnetic flux through the pickup coil 19. When the polarity ofthe alternating current supplied to the drive coil 17 is of the oppositepolarity, the magnetomotive force provided by the drive coil is inopposition to the permanent magnet field provided by core 20 so that thestrength of the magnet field threading the tines is substantiallyreduced and the tines are permitted to move away from each other and theassociated magnet cores. This increases the air gaps 211, 22, 23, and 24and, of course, the reluctance of the magnetic circuit carrying the fluxthreading the drive coil 16. The periodic increase and decrease of theflux threading the pickup coil 16 causes an AC. voltage to be inducedtherein, the frequency of which is controlled by and related to thenatural resonant frequency of the tuning fork. In this way anoscillatory circuit is established, the frequency of which is maintainedessentially constant by the action of the tuning fork, and thisoscillating circuit can be used to control a power amplifier (not shown)which may supply a constant frequency alternating current to the deviceor devices to be powered by the frequency regulated power supply systemas will be understood by those skilled in the art.

The tuning fork 10 and the associated base 11 are enclosed by a suitablecover 37 having an open bottom dimensioned to receive in close-fittingrelation the rectangular base 11. The cover is held in position by ascrew 38 which is received in the tapped hole 39 in the upper stud 27.In order to confine the magnetic flux emanating from the magnet cores l9and 20 and further to isolate the tuning fork from the influence of anystray magnetic fields, the cover 37 is preferably formed of a suitablemagnetically permeable material such as cold rolled stee. The leads 33and 35 extending from the pickup and drive coils l6 and 17 may beconveniently brought out from the base through a second groove in thebase indicated by number 4-0.

In addition to having the advantage of high efficiency, the tuning forkassembly forming the subject matter of this invention has the advantagethat it is easily and inexpensively constructed and further has smallphysical dimensions. By way of example, a 400 cycle tuning forkconstructed in accordance with the invention and which has been used tocontrol a power amplifier with a 5 watt output had physical dimensionsas follows: The tuning fork 10 was formed by bending a rectangular stripof material identified above as Ni-Span-C, the strip having a thicknessof .0225 inch and a width of .187 inch. When bent into a U-shape, thelength of the tines 12 and 13 as measured from the bottom of the curvedportion 14 to the outer extremity was 1.25 inches. The overall length ofthe tuning fork from the bottom of the base If to the top of the cover37 was 1.5 inches, and the lateral dimensions of the cover were 0.5 inchon each side.

In order to minimize the transmission of vibration energy from thetuning fork to the base, it is desirable to keep the area of the weldbetween the bottom of the curved portion 14 and the top of the dome 15as small as is consistent with the mechanical strength requirementsinvolved. In the tuning fork construction having the dimensions referredto above, satisfactory strength requirements were met with a spot welddiameter being maintained between the limits of .065 inch and .085 inch.Thus, the diameter of the weld was substantially less than the width ofthe tines. In addition to keeping the diameter of the weld as small aspossible, it is preferable to control the weld by selection of Weldingpressure and current intensity and duration so as to avoid the formationof fillets between the bottom of the curved portion 14 and the top ofthe dome 15 since it has been found that the presence of such filletsincreases the transmission of vibration energy from the fork to thebase. While welding is the preferred manner of rigidly connecting thefork to the base, other methods may be used such as soldering, brazing,cementing, etc.

Although this invention has been described by reference to particularembodiments thereof, it will be understood by those skilled inthe artthat numerous modifications and substitutions may be effected withoutdeparting either in spirit or scope from this invention in its broadestaspects.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. A tuning fork construction comprising a U-shtped strip of resilientmaterial having spaced, upstanding tines connected by a curved portionthe bottom of which curves upwardly, a base member having an upwardlyprojecting dome the sides of which curve downwardly from the apexthereof, the apex of said dome being in contact with the bottom of thecurved portion of the U-shaped strip intermediate the sides of theU-shaped strip and rigidly fastened thereto.

2. A tuning fork as set forth in claim 1 wherein the area of contactbetween the U-shaped strip and the apex of the dome is substantiallyless than the width of the U-shaped strip.

3. A tuning fork as set forth in claim 1 Wherein the area of contactbetween the U -shaped strip and the apex of the base member dome issubstantially less than the width of the strip and constitutes the onlycontact and support between the U-shaped strip and the base member tominimize the transmission of vibration energy from the tines to the basemember.

UNITED STATES PATENTS Haglund May 28, 1929 Karolus June 17, 1930Marrison May 2, 1933 Harris Aug. 5, 1941 Sebouh June 12, 1951 HetzelAug. 23, 1960 Gibbs Aug. 1, 1961

1. A TUNING FORK CONSTRUCTION COMPRISING A U-SHAPED STRIP OF RESILIENTMATERIAL HAVING SPACED, UPSTANDING TINES CONNECTED BY A CURVED PORTIONTHE BOTTOM OF WHICH CURVES UPWARDLY, A BASE MEMBER HAVING AN UPWARDLYPROJECTING DOME THE SIDES OF WHICH CURVE DOWNWARDLY FROM THE APEXTHEREOF, THE APEX OF SAID DOME BEING IN CONTACT WITH THE BOTTOM OF THECURVED PORTION OF THE U-SHAPED STRIP INTERMEDIATE THE SIDES OF THEU-SHAPED STRIP AND RIGIDLY FASTENED THERETO.